Dispersive kinetic model for the non-isothermal reduction of nickel oxide by hydrogen

Abstract

The kinetics of the non-isothermal reduction process of powder nickel oxide samples using hydrogen was investigated by temperature-programmed experiments at the different constant heating rates. The new procedure for the determination of density distribution function of activation energies (ddfEa), evaluated from the experimentally obtained non-isothermal conversion curves, was developed. The analytical relationships between the corresponding thermo-kinetic parameters for the investigated reduction process were established. From the influence of heating rate on the basic characteristics of ddfEa's, it was concluded that the evaluated ddfEa's are completely independent of the heating rate (vh). It was found that the value of activation energy at the peak of the distribution curve (Ea,max), at all considered heating rates, is in good agreement with the value of Ea,0 (96.6 kJ mol-1) calculated from the isoconversional dependence of activation energy, in the conversion range of 0.20≤α≤0.60. From the appearances of the true compensation effect, it was concluded that the factor that produces the changes of kinetic parameter values is a conversion fraction (α). Using the model prediction, the experimentally obtained conversion curves are completely described by the evaluated distribution curves (g(Ea)vhj) at all considered heating rates. It was concluded that the assumption about the distribution of potential energies of oxygen vacancies presented in NiO samples leads to the distribution of activation energies, which determine the kinetics of non-isothermal reduction processes. © 2008 Elsevier B.V. All rights reserved.